This invention relates generally to the processing of high molecular weight polymers to form compositions which are readily dissolvable and to uses for these compositions.
More specifically, this invention relates to a method for producing highly active, rapidly dissolving polymer compositions and to processes in which such polymer compositions find use.
High molecular weight, thermoplastic or viscoelastic polymers are notoriously difficult to dissolve without degradation and without significant reduction in molecular weight. It is not uncommon for such polymers to require several weeks of gentle agitation in a solvent to dissolve completely. Even then the concentration of polymer is such solutions is limited to a few percent at best because of the rapid increase in viscosity with increasing polymer concentration.
Dilute solutions of high molecular weight polymers in solvents such as hydrocarbons display unusual and useful flow characteristics. In particular, certain linear polymers, such as the high molecular weight alpha-mono olefinic polymers are noted for their effectiveness as drag reducing agents and as anti-misting agents. A drag reducing agent is a polymer which, when dissolved in a solvent, substantially reduces the friction loss during turbulent flow of the solution. An anti-misting agent is a polymer which, when dissolved in fuel, serves to significantly increase median droplet size and reduce flammability of fuel sprays caused by high velocity wind shear such as occurs during an aircraft crash landing.
A number of different approaches have been taken in the prior art to the problem of preparing large quantities of extremely dilute solutions--usually between five and about one hundred ppm--which are required for either drag reduction or anti-misting fuel use. For example, British Pat. No. 1,452,146 describes a method and apparatus for dissolving high molecular weight polymers on a large scale in solvents such as crude oil without significant polymer degradation. Patentees use a dissolving vessel having at least two compartments formed by a partition with provision for liquid communication between the compartments at the top and bottom of the partition. Polymer is introduced into moving or agitated solvent within the vessel to form a slurry. Agitation is accomplished by sparging gas into the bottom of one of the compartments to disperse the polymer particle throughout the solvent so as to prevent the polymer particles from agglomerating and to speed the dissolution process. Exemplary data set out in the patent shows the dissolving of crumb or cut polyisoprene of about 8 million molecular weight in crude oil at 32.degree. C. to form a solution of about 0.92% in about 120 to 160 hours. The molecular weight of the rubber was reduced by about 10% during the dissolution. The maximum concentration of polymer obtainable is determined by the viscosity of the final solution and, for high molecular weight polymers, maximum concentration ranges from about 0.5 to 2% by weight. The solution of polymer obtained is them metered into a flowing stream of crude oil to provide drag reducing effects.
A very different approach to the dissolving of high molecular weight polymers is shown by the Weitzen patent, U.S. Pat. No. 4,340,076. Weitzen found that high molecular weight polymers would very rapidly, almost instantaneously, dissolve in solvents for those polymers if the polymer was comminuted at cryogenic temperatures and the resulting polymer particles were introduced into the solvent without allowing them to warm. Polymer concentrations in the solution ranging from a few parts per million to 15% or more could readily be obtained. Essentially no polymer degradation, as indicated by a reduction in the molecular weight of the polymer, occurs during the dissolution.
Yet another approach to the preparation of dilute solutions of high molecular weight polymers is set out in the Mack patent, U.S. Pat. No. 4,433,123. Here, a solution of a high moleculuar weight polymer, suitable for use as a drag reducing agent or anti-misting agent, is produced by polymerization of an alpha-olefin in a hydrocarbon solvent. The entire mixture, containing polyolefin, solvent and catalyst particles, is used without separation to make up dilute solutions of the polymer in crude oil or other hydrocarbons.
This last approach, to use the entire reaction mixture of a polymerization process, is presently most favored from a commercial standpoint because of the great difficulties experienced in dissolving solid polymers without degradation. Drag reducing agents in field use today typically comprise a high molecular weight polymer dissolved in the polymerization solvent, which may be hexane or heptane, at a concentration ranging from a low of 2 to 3% to a maximum of 11 to 12%. These polymer solutions at typical commercial concentrations are thick, viscous liquids which are highly thixotropic and are also highly viscoelastic. They are commonly transported and stored in containers which can be pressurized with an inert gas to pressures of 30 to 70 psig in order to discharge the liquid from the container. Generally speaking, the lower the polymer concentration, the more rapidly and easily it will dissolve in crude oil or other liquids. The upper limit to polymer concentration is set by practical considerations including the need for an acceptably short dissolving time and the need to handle the concentrated polymer solution using readily available and reasonably priced equipment.
As may readily be appreciated, all three of the prior art approaches to the preparation of polymer solutions for drag reducing or anti-misting purposes have significant disadvantages. Dissolving solid polymer directly into crude oil or other hydrocarbons is very time consuming, requires large dissolving vessels because of the long dissolving time, and almost inevitably results in significant degradation of the polymer. The Weitzen dissolving process requires a source of liquid nitrogen for its operation. The present commercial approach, which utilizes the entire polymerization mixture, is unwieldy because the polymer solution must be transported and stored in pressure vessels such as those used to ship and store propane and butane. It also incurs a substantial financial penalty for transportation because little more than 10% of the mixture is polymer; the active ingredient in drag reducing or antimisting solutions. Also, the valuable solvent in which the polymer is dissolved cannot be recovered but instead becomes a part of the crude oil stream.
It is evident that a more highly concentrated, but easily handleable and fast dissolving, form of polymer would offer significant advantages over present systems in drag reduction and anti-misting applications as well as for a host of other uses.